The TOSLINK (EIAJ Optical, RC-5720C) optical S/PDIF audio connector.
On the back of the system, a variety of connectivity options were available such as analog audio, coaxial/XLR/toslink optical connectors for input and output, SCSI (for external hard drives or tape backup units), various sync sources (video/SMPTE/etc), MIDI, and RS-422 ports for remote controlling the Foundation 2000 from other devices, or using the it to control other devices.